1. Field of the Invention
The present invention relates to the drilling of boreholes and to survey and logging techniques used to determine the path and lithology of the drilled borehole. More particularly, but not exclusively, the invention is concerned with an improved system for sensing the inclination of a borehole formed by a drill bit rotated by a downhole motor, for telemetering borehole inclination and associated logging data to the surface while drilling, and for altering the drilling trajectory in response to the telemetered data.
2. Description of the Background
Drilling operators which power a drill bit by rotating the drill string at the surface have previously measured downhole parameters with sensors located closely adjacent the drill bit, and adjusted the drilling trajectory in response to the sensed information. U.S. Pat. No. 4,324,297 discloses strain gauges located directly above the drill bit to measure the magnitude and direction of side forces on the bit. The sensed information is transmitted to the surface by an electrical line, and the bit weight and rotational speed of the drill string may be altered in response to the sensed information to vary drilling trajectory.
In recent years, drilling operators have increasingly utilized downhole motors to drill highly deviated wells. The downhole motor or "drill motor" is powered by drilling mud pressurized by pumps at the surface and transmitted to the motor through the drill string to rotate the bit. The entire drill string need not be continually rotated, which has significant advantages over the previously described technique, particularly when drilling highly deviated boreholes. A bent sub or bent housing may be used above the drill motor to achieve the angular displacement between the axis of rotation of the bit and the axis of the drill string, and thereby obtain the bend to effect curved drilling. Alternatively, the angular displacement may be obtained using a bent housing within the drill motor, by using an offset drive shaft axis for the drill motor, or by positioning a non-concentric stabilizer about the drill motor housing. As disclosed in U.S. Pat. No. 4,492,276, a relatively straight borehole may be drilled by simultaneously rotating the drill string and actuating the downhole motor, while a curved section of borehole is drilled by activating the downhole motor while the drill string above the motor is not rotated. U.S. Pat. No. 4,361,192 discloses a borehole probe positioned within the drill pipe above a drill motor and connected to surface equipment via a wireline. The probe includes one or more accelerometers which measure orientation relative to the earth's magnetic field, and accordingly the probe is constructed of a non-ferromagnetic material. U.K. Patent 2106562 discloses a borehole probe which can be lowered on a wireline through a bore extending through a turbine of annular construction to a location between the turbine and the drill bit.
Significant improvements have occurred in measuring-while-drilling (MWD) technology, which allows downhole sensors to measure desired parameters and transmit data to the surface in real time, i.e., substantially instantaneously-with the measurements. MWD mud pulse telemetry systems transmit signals from the sensor package to the surface through the drilling mud in the drill pipe. Other MWD systems, such as those disclosed in U.S. Pat. Nos. 4,320,473 and 4,562,559, utilize the drill string itself as the media for the transmitted signals. U.S. Pat. No. 4,577,701 employs an MWD system in conjunction with a downhole motor to telemeter wellbore direction information to the surface, which is then used to control rotation of the drill string and activation of the downhole motor to effect a change in the borehole direction as previously described.
A downhole MWD tool typically comprises a battery pack or turbine, a sensor package, a mud pulse transmitter, and an interface between the sensor package and transmitter. When used with a downhole motor, the MWD tool is located above the motor. The electronic components of the tool are spaced substantially from the bit and accordingly are not subject to the high vibration and centrifugal forces acting on the bit. The sensor package typically includes one or more sets of magnetometers and accelerometers for measuring the direction and inclination of the drilled borehole. The tool sensor package is placed in a non-magnetic environment by utilizing monel collars in the drill string both above and below the MWD tool. The desired length of the monel collars will typically be a function of latitude, well bore direction, and local anomalies. As a result of the monel collars and the required length of the downhole motor, the sensor package for the MWD system is typically located from ten meters to fifty meters from the drill bit.
The considerable spacing between the MWD sensor package and the drill bit has long been known to cause significant problems for the drilling operator, particularly with respect to the measurement of borehole inclination. The operator is often attempting to drill a highly deviated or substantially horizontal borehole, so that the borehole extends over a long length through the formation of interest. The formation itself may be relatively thin, e.g. only three meters thick, yet the operator is typically monitoring borehole conditions or parameters, such as inclination, thirty meters from the bit. The substantial advantage of a real time MWD system and the flexibility of a downhole motor for drilling highly deviated boreholes are thus minimized by the reality that the sensors for the MWD system are responsive to conditions spaced substantially from the bit.
It is an object of the invention to provide an improved technique for accurately monitoring borehole conditions or parameters, such as borehole inclination, while drilling a deviated borehole utilizing a downhole motor.